[clang] Diagnose potential size confusion with VLA params [contd.] (PR #181550)

[Aaron Ballman via cfe-commits]

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@@ -1114,7 +1114,8 @@ def VexingParse : DiagGroup<"vexing-parse">;
 def VLAUseStaticAssert : DiagGroup<"vla-extension-static-assert">;
 def VLACxxExtension : DiagGroup<"vla-cxx-extension", [VLAUseStaticAssert]>;
 def VLAExtension : DiagGroup<"vla-extension", [VLACxxExtension]>;
-def VLA : DiagGroup<"vla", [VLAExtension]>;
+def VLASizeConfusion : DiagGroup<"vla-potential-size-confusion">;
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AaronBallman wrote:

Thinking out loud... Do we actually want two things?

1) `-Wchanges-meaning` implementation which diagnoses the declarations when their meanings change. This diagnostic would be tied to `-fexperimental-late-parse-attributes` for the cases discussed in this PR.
2) `-Wpotentially-confusing-use-of-identifier-thanks-to-lookup-rules` (let's pick a much better name for this, obviously) which diagnoses id expressions where the lookup rules find an unambiguous declaration but a human reader of the source code could reasonably expect a different lookup result. We have to be careful here though because this is not the expression form of `-Wshadow`. e.g., we would not want to diagnose this as potentially confusing because it's hard to argue that the lookup rules are unclear:
```
void func() {
  int n;
  {
    int n; // Shadow warning makes sense
    n = 12; // Not confusing as an expression
  }
};
```
so we'd have to figure out what rules we use to determine whether something is potentially confusing or not.

https://github.com/llvm/llvm-project/pull/181550